Circuit arrangements for high frequency-operation of low-pressure discharge lamps are known and the referenced Fahnrich and Zuchtriegel patents disclose various circuits of this type. Operating discharge lamps, typically fluorescent lamps, with high-frequency supply, permits substantial improvements in the operating condition of the lamps, for example, better ignition, less flicker, and higher light output when compared with operation of the lamps directly from a power network. Additionally, the physical dimensions of auxiliary or accessory or ballast can be substantially reduced. Such operating circuits, however, require more complex networks and circuits, particularly in order to insure that there will be sufficient suppression of radio interference noise. The circuits should be so arranged that the current to be supplied from the power network will be as close to sinusoidal as possible, with the highest possible power factor, preferably, a power factor of one or as close to it as possible.
The referenced U.S. Pat. No. 5,008,597, Zuchtriegel, the disclosure of which is hereby incorporated by reference, assigned to the assignee of the present application, describes a suitable circuit for operation of a fluorescent lamp. This circuit uses a half bridge inverter or oscillator, having two alternatingly switching transistors. The center terminal between the two transistors is coupled to a series resonance circuit, formed of a resonance inductance, a coupling capacitor, and resonance capacity. The low-pressure discharge lamp is integrated into the series resonance circuit. The patent further discloses an active harmonic filter which insures that the power supplied will have effectively sinusoidal wave shape, conforming to standards by the International Electrotechnical Commission (IEC). The harmonic filter is formed by four diodes, connected similarly to a bridge rectifier, and integrated-in the circuit. The diodes are coupled in dc forward conductive direction between the dc output of a network power supply rectifier and a positive terminal of a smoothing capacitor for supplying the inverter and forming an output from the rectifier. The four diodes of the harmonic filter interrupt transport of charge to the smoothing capacitor in synchronism with the switching of the inverter transistors. Switching of the diodes is controlled by the diodes themselves, connected in series with respect to each other, by terminals connected to the center tap between the serially connected diodes. The center tap or connection of a first diode pair is connected directly to the center tap of the half bridge inverter through a pumping or auxiliary capacitor and, further, connected through a further auxiliary or pumping capacitor between the resonance inductance and the coupling capacitor to a junction in the series resonance circuit. The center tap or connection of the second diode pair is connected through a dc blocking capacitor and an inductance to a junction or terminal in the series resonance circuit.
This network functions well and draws approximately sinusoidal current from the network with a network power factor about or somewhat larger than 0.9.